fix(checkbox): Delay checked-unchecked fade

[patrickrodee]

Delay the fade of the checkbox background by 90ms during the transition from checked to unchecked. This makes the checked to unchecked animation a better inversion of the unchecked to checked animation.

[patrickrodee]

Before

Notice how when going from checked to unchecked the background fade starts at the same time as the check transition.

After

Notice how when going from checked to unchecked the background fade starts most of the way through the check transition.

[codecov-io]

Codecov Report

Merging #2108 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2108   +/-   ##
=======================================
  Coverage   99.43%   99.43%           
=======================================
  Files          84       84           
  Lines        3720     3720           
  Branches      485      485           
=======================================
  Hits         3699     3699           
  Misses         21       21

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